Error adding MX record

I'm running ISPConfig 2.2.13 on Debian Sarge. I've got several hundred customers/domains running, for the most part smoothly.

One of my customers is asking that I modify his MX records to point to a machine other than ISPConfig. We are doing this for several other customers as well - we offer a service called Premium Spam Control, and several of our customers have their e-mail filtered through PSC.

When configuring mail this way, we set the MX records to point to rb.<our domain>.com and fr.<our domain>.com. No problem - as I said, we are doing that for several customers. This latest customer, though, is a problem. When I set up an MX record for fr at priority 30, the record is accepted. When I attempt to add an MX record for rb at priority 20, I get an error reading " This or a similar MX Record does already exist in this zone." Examining the MX records for this domain does not show any MX records other than the one pointing to fr. I have emptied the Admin recycle bin and the reseller level recycle bin.

I tried looking through the database records, but I haven't yet figured out which tables link the dns_mx table to the other required tables.

Can someone document for me how the dns_mx table links to the other tables that get involved here? I'm familiar with tracing info through isp_isp_domain, isp_dep, etc, I'm just not sure what tables to use and what fields tie them together in the case of DNS issues.

So, my confusion at present is why DNS Manager only shows one MX record. I understand why I can't add a new MX record pointing to Rb; it's because there's already one in the database.

For now I'm working around this issue by manually editing the customer's zone file to add the MX record for Rb. But I'd like to figure out why this is happening, in case it manifests itself on another domain in the future. Any ideas?